This invention relates to optical communication systems and, in particular, to a transmitter for digitally modulated RF sub-carriers.
An ever increasing communication need of today is to deliver multimedia services such as voice, data, high speed internet access, video conferencing, video on demand, and broadcast television video to small businesses and residences. Cost is the prominent issue for the deployment of such networks. Optical fiber extending closer to usersxe2x80x94Fiber to the curb (FTTC) and hybrid fiber coaxial (HFC), or fiber all the way to the userxe2x80x94Fiber to the Home (FTTH)xe2x80x94are the technologies currently being deployed to meet present and future needs. Both the existing operators and overbuilders are taking fiber as deep into their networks and closer to the customers as their costs allow.
Two different optical fiber communication systems have evolved for carrying information to homes and businesses. Each system has its own specialized equipment, its own physical plant and its own standards. One system delivers information by a digitally modulated series of light pulses. These are referred to as baseband signals. FIG. 1 illustrates a simplified baseband modulation scheme. Typically a digital 1 is represented by a light pulse in the series and a digital 0, by the absence of a pulse in a pulse position. Alternatively, the signal can be inverted with a pulse representing digital 0 and its absence representing 1.
A second system uses a plurality of radio frequency separated carriers. Each carrier is modulated to transmit a higher order digital signal. These are passband signals. FIG. 2a schematically illustrates a passband lightwave transmission system 20 comprising a hub 21, and a plurality of fibers 22A, 22B, 22C connecting the hub to a respective plurality of fiber nodes 23A, 23B and 23C. Each node is connected, as by a plurality of fibers or coaxial cables 24A and 24B to a plurality of homes 12 and businesses 13.
FIG. 2b illustrates the radio frequency spectrum of a typical digitally modulated passband signal. The signal comprises a plurality of different radio frequency (RF) carriers spaced apart in frequency (e.g. 6 MHz spacing in the NTSC system). Each of the carriers is modulated among a plurality of states to carry a higher order digital signal to encode plural bits for each modulation state. The modulation can be amplitude modulation, frequency modulation, phase modulation or a combination of them.
Digital passband signals are conventionally transmitted using two RF carriers that are frequency locked but 90 degrees out of phase. The two carriers are said to be in quadrature. The two carriers are separately amplitude modulated (AM), and the modulated carriers are combined to form a single RF output having both amplitude information corresponding to their vector sum and phase information corresponding to their vector angle. The technique is known as quadrature amplitude modulation or QAM.
FIG. 2c illustrates the simplest case of QAM, which occurs when each of the carriers has only two states (e.g. +V and xe2x88x92V). One carrier is considered the reference carrier and is called the in-phase channel. It""s amplitude is represented along the horizontal axis of FIG. 2c. The other carrier, 90xc2x0 out of phase, is called the quadrature channel. Its amplitude is represented along the vertical axis. As can be seen from the diagram, if each carrier has two states (xc2x1V, xc2x1V), then there are four possible combined outputs, each of which can represent two bits of information: (0,0), (0,1), (1,0), (1,1). This simple modulation scheme is known as quadrature phase shift keying (QPSK).
Similar modulation schemes can be based on amplitude modulation of the carriers among a larger number of states. For example if both carriers can be modulated among four amplitudes, the combined output can represent 4xc3x974=16 states, and the modulation is called 16-QAM. Modulation using 8xc3x978=64 states is 64-QAM. In an optical communication system, optical transmission of sub-carrier multiplexed (SCM) multi-channel M-ary quadrature amplitude modulation (QAM) signals has many advantages over the analog amplitude modulated vestigial side band (AM-VSB) signals. Some of the advantages include: requirements of lower carrier signal-to-noise ratio (CNR); less sensitivity to nonlinear distortion; high spectral efficiency; high system transmission capacity; and, the ability to transmit all multimedia services (telephony, digital video and data). Cable companies are upgrading their Hybrid Fiber Coax (HFC) networks to create a fully interactive two-way network to carry high bandwidth multimedia services into and out of homes. Because of these advantages coupled with higher revenue generating opportunities for service providers, arrival of High Definition Television (HDTV), availability of digital televisions and set top conversion boxes, transmission of video signals in near future is likely to be all-digital.
While data can advantageously be transmitted using a baseband technique, for video transmission a passband technique is preferred by both telecom and cable TV industry as well as by overbuilders because of technical, economic and management reasons. For these reasons the full service access network (FSAN) group of global telecommunication operators has recently drafted new standards, called G983.wdm, for ITU-T for adding digital video in passband to ATMPON baseband services utilizing frequency division multiplexing (FDM) and wavelength division multiplexing (WDM) techniques which is shown in FIG. 3. Referring to FIG. 3 there is shown the architecture of an exemplary WDM network to deliver digital video in passband over a separate wavelength along with baseband data using wavelength division multiplexing (WDM). Digital video can be delivered by utilizing QPSK in 950-2050 MHz band or 64-QAM in MMDS 216-422 MHz band or CATV 550-800 MHz band. Located at the central office 302 is a data optical line terminator 304, a video optical line terminator 306 and a wavelength division multiplexer 308. The data optical line terminator 304 and the video optical line terminator 306 are coupled to the wavelength division multiplexer 308. The wavelength division multiplexer 308 is coupled to an optical fiber transmission link 310. The optical fiber transmission link 310 is coupled to a 1:n splitter 312, which splits the optical signal for delivery to a home 314. Located at the home 314 is a wavelength division demultiplexer 316, a data optical network terminator 318, and a video optical network terminator 320. The data optical network terminator 318 and the video optical network terminator 320 are coupled to the wavelength division demultiplexer 316. The wavelength division demultiplexer 316 is coupled to the 1:n splitter 312.
One of the more expensive network elements in the video lightwave transmission system is the video laser transmitter. For a low cost per user, a video transmitter needs to be shared by as many users as possible. If by suitably designing a laser transmitter, the optical/electrical (O/E) receiver sensitivity can be increased by xcx9c4 dBo, two and half times more users can share the same transmitter. Note that dBo represents optical dB, dBe represents electrical dB, and dBm represents power with reference to 1.0 mW. Thus dBmo and dBme will represent optical and electrical dB with reference to 1.0 mW optical and electrical power, respectively.
Presently CATV and MMDS utilize a 64-QAM format for downstream services. QPSK (4QAM) or 16 QAM formats are used for upstream services typically utilizing a 1.3 xcexcm Febry-Perot laser. When the value of M in M-ary increases by one, for a given bit error rate the required carrier signal to noise ratio (CNR) for a sub-carrier at the receiver increases by 3-dBe which is equivalent to 1.5 dBo decrease in receiver sensitivity. A 4 dBo increase in receiver sensitivity will allow 16-QAM and 64-QAM schemes to be upgraded to 64-QAM for upstream and 256 QAM for downstream, respectively within the same power budget. This upgrade results in a higher transmission capacity.
In one aspect, the present invention features a transmitter for digitally modulated RF sub-carriers comprising an electroabsorption modulator integrated distributed feedback laser (EML) having an electroabsorption modulator section for transmitting the digitally modulated RF sub-carriers in the sub-octave frequency range. Wherein the negative bias is selected to increase modulation index of the electroabsorption modulator section of the electroabsorption modulator integrated distributed feedback laser above a modulation index of an electroabsorption modulator section of an electroabsorption modulator integrated distributed feedback laser having a zero bias, without causing in-band non-linear distortion.